Telecom network architechture






Architecture:

Mobile Station (MS):
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
Base Station Subsystem (BSS):
Base Transceiver Station (BTS)
Base Station Controller (BSC)
Network Switching Subsystem (NSS):
Mobile Switching Center (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)


Mobile Equipment:
a.       Portable, vehicle mounted, hand held device. 
b.      Uniquely identified by an IMEI (International Mobile Equipment Identity).
c.       Voice and data transmission.
d.      Monitoring power and signal quality of surrounding cells for optimum handover.
e.      Power level: 0.8W – 20 W.
f.        160 character long SMS.


Subscriber Identity Module (SIM):
a.       Smart card contains the International Mobile Subscriber Identity (IMSI).
b.      Allows user to send and receive calls and receive other subscribed services.
c.     Encoded network identification details - Key Ki, Kc and A3, A5 and A8 algorithms.
d.    Protected by a password or PIN.
e.    Can be moved from phone to phone – contains key information to activate the phone.

Base Transceiver Station (BTS):
a.       Encodes, encrypts, multiplexes, modulates and feeds the RF signals to the antenna.
b.      Frequency hopping.
c.       Communicates with Mobile station and BSC.
d.      Consists of Transceivers (TRX) units.

Base Station Controller (BSC):
a.       Manages Radio resources for BTS.
b.      Assigns Frequency and time slots for all MS’s in its area.
c.       Handles call set up.
d.      Trans coding and rate adaptation functionality.
e.      Handover for each MS.
f.        Radio Power control. 
g.       It communicates with MSC and BTS.

Mobile Switching Center (MSC):
a.       Heart of the network.
b.      Manages communication between GSM and other networks.
c.       Call setup function and basic switching.
d.      Call routing.
e.      Billing information and collection.
f.     Mobility management
                                - Registration
                                - Location Updating
                                - Inter BSS and inter MSC call handoff
g.    MSC does gateway function while its customer roams to other network by using HLR/VLR.

GMSC (Gateway MSC):
Handles all traffic to and from GSM and external circuit switched networks, such as PSTN, ISDN or other mobile networks.

SGSN (Serving GPRS Support Node and GGSN):
Gateway GPRS Support Node has similar functionality as MSC / GMSC, but for the packet switched part of the network. GGSN handles connections to external IP networks.

Home Location Registers (HLR):
a.       Permanent database about mobile subscribers in a large service area(generally one per GSM network operator).
b.      Database contains IMSI (International Mobile Subscriber Identity), MSISDN (Mobile Subscriber Integrated Services Digital Network), roaming restrictions, and supplementary services.

MSISDN (15 Digits):
Country Code [1-3]
National Mobile Number


IMSI (15 Digits):
Mobile County Code (MCC) [3]
Network Code (NC) [2-3]
Subscription Identification


Visitor Location Registers (VLR):
a.       Temporary database which updates whenever new MS enters its area, by HLR database.
b.      Controls those mobiles roaming in its area.
c.       Reduces number of queries to HLR.
Database contains IMSI, TMSI (Temporary Mobile Subscriber Identity), MSISDN, MSRN (Mobile Station Roaming Number), Location Area (LA), authentication key.

Authentication Center (AUC):
a.       Protects against intruders in air interface.
b.      Maintains authentication keys and algorithms and provides security triplets (RAND [Non predictable random number], SRES [Signed response], Kc [Ciphering key]).
c.       Generally associated with HLR.

Equipment Identity Register (EIR):
a.       Database that is used to track handsets using the IMEI (International Mobile Equipment Identity).
b.      Made up of three sub-classes: The White List (IMEI, assigned to valid ME), The Black List (IMEI reported stolen) and the Gray List (IMEI having problems like faulty software, wrong make of equipment etc).
c.       Only one EIR per PLMN (Public Land Mobile Network).

GPRS support nodes (GSN):
A GSN is a network node which supports the use of GPRS in the GSM core network. All GSNs should have a Gn interface and support the GPRS tunneling protocol. There are two key variants of the GSN, namely Gateway and Serving GPRS support node.

Gateway GPRS support node (GGSN):
The gateway GPRS support node (GGSN) is a main component of the GPRS network. The GGSN is responsible for the internetworking between the GPRS network and external packet switched networks, like the Internet and X.25 networks.
From an external network's point of view, the GGSN is a router to a "sub-network", because the GGSN ‘hides’ the GPRS infrastructure from the external network. When the GGSN receives data addressed to a specific user, it checks if the user is active. If it is, the GGSN forwards the data to the SGSN serving the mobile user, but if the mobile user is inactive, the data is discarded. On the other hand, mobile-originated packets are routed to the right network by the GGSN.
The GGSN is the anchor point that enables the mobility of the user terminal in the GPRS/UMTS networks. In essence, it carries out the role in GPRS equivalent to the home agent in Mobile IP. It maintains routing necessary to tunnel the protocol data units (PDUs) to the SGSN that services a particular MS (mobile station).
The GGSN converts the GPRS packets coming from the SGSN into the appropriate packet data protocol (PDP) format (e.g., IP or X.25) and sends them out on the corresponding packet data network. In the other direction, PDP addresses of incoming data packets are converted to the GSM address of the destination user. The readdressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN address of the user and his or her profile in its location register. The GGSN is responsible for IP address assignment and is the default router for the connected user equipment (UE). The GGSN also performs authentication and charging functions.
Other functions include subscriber screening, IP pool management and address mapping, QoS and PDP context enforcement.
With LTE scenario the GGSN functionality moves to SAE gateway (with SGSN functionality working in MME).

Serving GPRS support node (SGSN):
A serving GPRS support node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI, address used in the packet data network) of all GPRS users registered with it.

Common SGSN functions:
a.       Detunnels GTP packets from the GGSN (downlink).
b.      Tunnel IP packets toward the GGSN (uplink).
c.       Carry out mobility management as Standby mode mobile moves from one Routing Area to another Routing Area.
d.      Billing user data.

GSM/EDGE specific SGSN functions:
Enhanced Data Rates for GSM Evolution (EDGE) specific SGSN functions and characteristics are:
a.       Maximum data rate of approx. 60 kbit/s (150 kbit/s for EDGE) per subscriber.
b.      Connect via frame relay or IP to the packet control unit using the Gb protocol stack.
c.       Accept uplink data to form IP packets.
d.      Encrypt down-link data, decrypt up-link data.
e.      Carry out mobility management to the level of a cell for connected mode mobiles.

WCDMA specific SGSN functions:
a.       Carry up to about 42 Mbit/s traffic downlink and 5.8 Mbit/s traffic uplink (HSPA+).
b.      Tunnel/detunnel downlink/uplink packets toward the radio network controller (RNC).
c.       Carry out mobility management to the level of an RNC for connected mode mobiles.

Access point:
An access point is:
a.       An IP network to which a mobile set can be connected.
b.      A set of settings which are used for that connection.
c.       A particular option in a set of settings in a mobile phone.
d.      When a GPRS mobile phone sets up a PDP context, the access point is selected. At this point an Access Point Name (APN) is determined.

Example: aricenttechnologies.mnc012.mcc345.gprs, Internet, mywap, hcl.cisco.ggsn.

This access point is then used in a DNS query to a private DNS network. This process (called APN resolution) finally gives the IP address of the GGSN which should serve the access point. At this point a PDP context can be activated.

PDP context:
The packet data protocol (PDP; e.g., IP, X.25, Frame Relay) context is a data structure present on both the serving GPRS support node (SGSN) and the gateway GPRS support node (GGSN) which contains the subscriber's session information when the subscriber has an active session. When a mobile wants to use GPRS, it must first attach and then activate a PDP context. This allocates a PDP context data structure in the SGSN that the subscriber is currently visiting and the GGSN serving the subscriber's access point. The data recorded includes

a.       Subscriber's IP address.
b.      Subscriber's IMSI.
c.       Subscriber's
-Tunnel Endpoint ID (TEID) at the GGSN
-Tunnel Endpoint ID (TEID) at the SGSN

The Tunnel Endpoint ID (TEID) is a number allocated by the GSN which identifies the tunnelled data related to a particular PDP context.
Several PDP contexts may use the same IP address. The Secondary PDP Context Activation procedure may be used to activate a PDP context while reusing the PDP address and other PDP context information from an already active PDP context, but with a different QoS profile. Note that the procedure is called secondary, not the resulting PDP contexts that have no such relationship with the one the PDP address of which they reused.
A total of 11 PDP contexts (with any combination of primary and secondary) can co-exist. NSAPI are used to differentiate the different PDP context.

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